The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, to the modulation of x-ray dose applied to a patient during a scan.
In a computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the "imaging plane." The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a "view" and a "scan" of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Quantum noise degrades the diagnostic quality of a CT image and this noise is related to the amount of x-rays, or "dose", employed to acquire the attenuation measurements, and to the attenuation characteristics of the patient. Image artifacts due to noise will increase if the x-rays measured at the detector drop to low levels either because the prescribed x-ray dose is too low or the beam is highly attenuated by patient anatomy. The x-ray dose is controlled during the exposure by the emission current ("mA") which flows in the x-ray tube, and the practice in the past has been to fix this current at a level which provides a constant dose during the entire scan. However, more recently the x-ray dose has been varied during the scan by modulating the x-ray tube current from slice-to-slice and during the acquisition of each slice. This method is described for example in co-pending U.S. patent application Ser. No. 155,037 filed on Nov. 19, 1993 and entitled "Variable Dose Application By Modulation of X-Ray Tube Current During CT Scanning" U.S. Pat. No. 5,319,333.
There are a number of practical problems in implementing variable dose through tube current modulation. First, to be effective, the x-ray tube current must be modulated at a rate which is very demanding on the x-ray controller. In addition, safety requires that the x-ray dose be carefully monitored and this is complicated when the dose is continuously modulated throughout the scan.